The objective of this study is to assess the clinical safety and tolerance of ARI-509, a new aldose reductase inhibitor, in diabetic patients; to assess the pharmacokinetics of ARI-509 after multiple doses by measuring the plasma, erythrocyte (RBC) and sural nerve concentrations of ARI-509; and to assess preliminary pharmacodynamics of ARI-509 by serial measurement of sorbitol, fructose and glucose levels.